1. Field of the Invention
The present invention generally relates to function generators for producing an output signal which is a predetermined function of an input signal or signals and particularly to function generators utilizing a cam following servomechanism to produce the desired functional relationship between the input and output signals.
2. Description of the Prior Art
Prior art pneumatic controllers are known which provide a manual adjustment of the controller gain to produce an output signal which is linearly proportional to an input signal. Function generators are also known which use such controllers in conjunction with pneumatic servomechanisms to vary the gain of the controller according to the input signal to thereby produce an output signal according to a function determined by the servomechanism and the controller input signal. An example of such a function generator may be seen in U.S. Pat. No. 3,404,604.
The prior art function generators referred to above utilized a mechanical cam follower which physically engaged the contour of a preformed rigid cam member to set the gain of the controller according to the contour of the cam member. The cam member was made rotatable to provide a different contoured surface to the cam follower for different input signals. The function generator thereby was able to produce an output signal which varied as a predetermined function of the input signal. Different contoured cams provided different functional relationships. However these function generators had certain limitations arising mainly from the use of the mechanical cam follower.
In order for the cam member to provide repeatability and long life performance it had to be constructed from rigid metal materials which would not deteriorate from extended movable contact with the mechanical cam follower. This made the forming of the requisite functional cam contours difficult since the cam member had to be stamped or otherwise cut on industrial machinery. Any custom fitting of the requisite functional contour for a particular unit in the field was thus difficult if not impossible.
Further, there were limitations due to the contact of the follower to the edge of the cam by way of a roller: First, the roller was unable to follow a concave cam contour whose radius of curvature was smaller than that of the roller. Second, the roller could not be caused to roll up a step change in the cam contour.
A further limitation was imposed by the placement of the servo summing station ahead of the cam member. This placement required that any increased input signal equilibrium could only be reached by having the cam member shaped to provide a continuously increasing negative feedback signal. In practice this prevented the cam from being shaped with a contour that anywhere approached a constant radius and also prevented the cam contour from reversing its slope.
The foregoing problems associated with the prior art devices and others are solved by the present invention which will be generally described next.